Life in the fast lane: one of New York's iconic yellow taxis. Photograph: Jan Johannessen/Getty Images

I once heard somebody say that signing tours could kill you faster than drink, drugs and fast women; regrettably my experience in at least two of those categories is limited, but during my recent mini-tour of New York and Chicago it nearly came true. My assistant Rob accompanied me to New York ComicCon, which was frenetic to say the least, but even when you are jetlagged it's all in a day's work; we've done this many times before. There is such a thing as signing tour machismo.

It was a great event, especially meeting the actor Sean Astin and catching up after a few years. So the day passed noisily, but that was only one day down and then there was business to deal with at my US publishers before a talk at Barnes & Noble and then more publishing business and interviews and so on, after which we had a free day before heading off to Chicago. Rob suggested we go and pay our respects at Ground Zero. Unfortunately both of us had gone down with a little bit of food poisoning, or so it seemed, but it appeared to have gone away after lunch and so we got in a cab for an extremely bumpy ride.

We headed south through Manhattan, both feeling absolutely dreadful. We arrived at the foot of Freedom Tower and had enough time for one single photograph before I decided to turn back, as I was feeling so nauseous. We got back into the very same cab and began the trek back to 52nd Street. However, we were only five minutes into the journey when, according to Rob, my breathing became very laboured. I felt very cold, although sweat was pouring down my face; I couldn't focus and just seemed to be slipping away. There was nothing I could get a grip on. Rob kept asking me if I was OK and assuring me we didn't have far to go … the little liar! We still had a good 15 minutes in that bumpy cab, and I have to take his word for what happened next; I collapsed back into the seat and, again according to Rob, was now definitely in a very bad way. But chalk one up for the boy scouts and their first aid training, because he grabbed me and cleared my airways – no task for the squeamish – while yelling at the cabbie to drive faster.

By the time we got back to our hotel I was conscious enough to decide that this was just one of those things and insisted that a lie down would do the trick. However, my young-adult editor had already called the doctor, and filled me up with Pringles and vitamin drinks while we waited; and since they were worried, I was worried too. By the time the doctor arrived I felt fine again, but he insisted on giving me a good checking over. And good job he did, because when he took my pulse he felt an irregular heartbeat and immediately packed us all off to the nearest hospital. There I was warned that I might quite possibly remain for a few weeks, as a worst-case scenario, as they couldn't possibly recommend that I fly in that state.

I was suddenly covered in miscellaneous pipes and electrodes. I was impressed by the thoroughness of it all and even more impressed when towards six in the morning they declared that as my heart had spontaneously returned to a normal rhythm as far as they were concerned I could leave, with a stern instruction to see my specialist back in England as soon as possible. I did so and it turned out that I had low blood pressure, probably exacerbated by the circumstances of the signing tour, odd hours, jet lag, the irregular meals and general rushing about. Nevertheless, on the day after leaving the hospital we flew on to Chicago, where we did an event at Anderson's bookshop – one of the best there is – and we had a great crowd, none of whom would have known that there was anything wrong. The show must go on.

The flight home was bearable, but I started thinking to myself, "Look, you are in your mid-60s, with stents in your heart and a daily pharmaceutical regime in a myriad glowing colours. And only a few months ago you were charging through a bog in Borneo in search of a lost orangutan." I remembered the days when I used to fly around the US with nothing more than a transparent plastic bag, a mobile phone, a wallet, yesterday's washing and a friendly grin for every homeland security officer – necessary because authors on signing tours don't have the same footprint in the eyes of security professionals as real people; we tend to have a lot of one-way tickets. My perfectly transparent bag also worried every single one of them, because I wasn't carrying much clothing. (As recommended by Neil Gaiman, I had adopted the sensible routine of buying fresh clothes as required and then giving to the helpful escorts who attend every author when they arrive in a new city a small bag of used clothing and the money to post it back to the UK. Some of the nicest ones actually washed them before doing so!)

When I put it like that, it seemed totally mad. Fun, but mad. Perversely, it was a great life – it still is on the whole – and I wish to keep it like that for as long as possible. Right now, I see the calendar filling up and note that next year I will already, among other things, be doing a tour of Australia and New Zealand, attending the next American Discworld convention in Baltimore and a host of lesser media engagements. It is amazing how many people want me to do something that will take just "a moment of my time", which invariably takes more than a week, and I suspect this is the same for almost all authors.

Now that I have been made painfully aware of the ticking clock, and the possibility of an erratically ticking heart, tiny voices are saying things like, "You damn fool! You could be sitting at home in the chapel, happily writing books and not worrying your wife too much and staying within easy reach of a surgery and a pretty good hospital." It's a thought, I suppose – and I will respect the advice of my medics.

And the clock ticks...

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